Grit is a fine, gravelly substance sometimes offered to pet birds as a dietary supplement. When a bird ingests grit, it makes its way to the gizzard where it helps grind food into particles that are easy to absorb, and aids in digestion.

Azolla is rich in proteins and is really good for ducks, chickens.

Grit helps in digestion to grind their food up and is very important. I think they get enough from the driveway and in the yard. :)

I give mine crushed oyster shell, about once a month. Great source of calcium for egg production too. Giving azolla increases the weight of broiler chickens..

As I was researching on raising duck, this is what I read about grits in an another site and found useful:

"Grit is small bits of rock that birds consume. The grit lodges in the gizzard to enable the bird to grind up whole grains. If the birds are outside, they are finding all the grit they need. If you are feeding them mash, crumbled or pelleted feed, grit is not necessary as the feed has already been ground. If you do decide to feed grit, do not provide it until the birds are a month old and do not mix it in the feed. Just provide it in a separate feeder so they can eat what they want."